Alumina-based fused grain

Abstract

The invention relates to a fused grain having: - a chemical composition such that, in percentages by weight on the basis of the oxides: 0.30% ≤ MgO ≤ 2.30%, Cr2O3 < 0.2%, Na2O < 0.1%, oxides other than MgO, Cr2O3, Na2O and Al2O3 < 1.5%, Al2O3: remainder to 100%; - a crystallographic composition characterized by an X-ray diffraction pattern having a first peak in the 2θ angle range of between 17° and 18°, and optionally a second peak in the 2θ angle range of between 43° and 44°, or "corundum phase peak", each peak having a relevant surface area, in counts x degrees, the RI ratio of the surface area of the first peak to the sum of the surface area of the first peak and the surface area of the corundum phase peak being greater than 0.02.

Description

[0001] Description

[0002] Title: Alumina-based fused grain

[0003] technical field

[0004] The present invention relates to a fused alumina-based grain, a mixture of said grains, a method for manufacturing said mixture of grains, and an abrasive tool comprising said mixture of grains. The invention further relates to uses of the grains according to the invention for abrading a surface made of hardened steel.

[0005] Previous art

[0006] Abrasive tools are generally classified according to the conditioning of their abrasive grains: loose abrasives (powders of grains not fixed to a support, used by projection or suspension), coated abrasives (support such as cloths or papers, on which the grains are arranged in a few layers) and bonded abrasives (circular grinding wheels, sticks, etc.).

[0007] In bonded abrasives, the abrasive grains are pressed together with an organic or vitreous binder, typically a binder composed of oxides, primarily silicates. The abrasive grains themselves must possess good mechanical properties against abrasion and exhibit good mechanical cohesion with the binder; that is, the interface with the binder must be solid.

[0008] Among abrasive grains, a distinction is made between fused grains and sintered grains, which exhibit different microstructures. The problems posed by sintered and fused grains, and the technical solutions adopted to address them, are therefore generally different. A composition developed to manufacture a fused grain is thus not necessarily applicable to manufacturing a sintered grain with the same properties, and vice versa.

[0009] In the field of fused grains, alumina and zirconia-based materials have been known since US-A-3, 181,939. These grains are generally composed of 10 to 60% zirconia, 0 to 10% of an additive, with alumina being the remaining component. In practice, the zirconia content of commercial products is either around 25% or around the alumina-zirconia eutectic value, which is approximately 42% zirconia, generally between 35 and 50%, as described in US-A-3, 891,408. US-A-4,457,767 also describes fused grains. For example, the grains marketed by Saint-Gobain (France) under the name NZPlus® can be cited. These grains typically contain 39% by mass of zirconia and 0.8% of Y2O3, less than 0.5% impurities, the remainder being alumina.Mixtures of these grains are widely used for coated abrasives or for organic bonded abrasive wheels, particularly in operations with high material removal rates (roughing, cutting, etc.), especially on stainless steel.

[0010] The Applicant has developed many types of fused alumina-based abrasive grains containing magnesia MgO.

[0011] In particular, he developed slowly cooled, molten alumina grains, called MA88K, marketed by Saint-Gobain Speciality Grains and Powders, and corundum-based molten grains described in FR2853898A1, which contain between 1.5% and 6.5% MgO, or in W02004 / 094554, which contain between 2.2% and 6.5% MgO, as a mass percentage based on the oxides. These latter grains are manufactured by melting raw materials. The molten material is then rapidly cooled to promote the formation of fine, oriented structures, for example, using a casting device between thin metal plates such as that described in US patent 3,993,119. The cooled material is finally ground, for example by means of roller mills, then sieved and classified into series of particle size distributions, or "grits", meeting precise standards, for example FEPA.

[0012] The Applicant has also developed grains, described in application WO2023111156, which have an MgO content of 2.5% to 5.8%, but also 0.2% to 4.5% of C Oa, which increases the manufacturing cost.

[0013] The Applicant has further developed grains with an MgO content of 4.8% to 7.2%, as described in the co-dependent application FR2306000. Advantageously, unlike the grains described in WO2023111156, these grains contain substantially no CO₂. These grains require additional calcination heat treatment for abrasion of hardened steel. Finally, their high MgO content makes grain manufacturing more complex and increases their cost.

[0014] The Applicant has further developed fused alumina grains coated at least partially with a silica-containing coating. The addition of a silica-containing coating also increases manufacturing costs. There is an ongoing need to improve abrasive grains that contain substantially no Cr₂O₃ and that, for the abrasion of a hardened steel surface, exhibit a higher efficiency than prior art grains and a specific energy substantially equal to or lower than that of prior art grains.

[0015] One aim of the invention is to meet, at least partially, this need.

[0016] Summary of the invention

[0017] According to a first principal aspect of the invention, this goal is achieved by means of a molten grain having:

[0018] - a chemical composition such that, in mass percentages based on oxides:

[0019] 0.30% < MgO < 2.30%,

[0020] Cr2O3 < 0.2%,

[0021] Na2O < 0.1%,

[0022] Oxides other than MgO, Cr2O3, Na2O and Al2O3 < 1.5%,

[0023] A12O3: complement to 100%;

[0024] - a crystallographic composition characterized by an X diffraction diagram showing a first peak in the angular range 29 between 17° and 18° and, optionally, a second peak in the angular range 29 between 43° and 44°, or "corundum phase peak", each peak having a respective area, the area being in counts x degrees (counts times degrees), the ratio RI of the area of ​​the first peak to the sum of the area of ​​the first peak and the area of ​​the corundum phase peak being greater than 0.02.

[0025] This RI report is designated by "DRX1 signature".

[0026] The performance obtained with molten grains according to the first aspect of the invention proved to be exceptional.

[0027] Without being bound by this theory, the inventors discovered that the DRX1 signature above, which appears to correspond to the presence of a crystalline phase comprising the elements aluminium, magnesium and oxygen and exhibiting, on an X diffraction diagram, a diffraction peak whose maximum intensity is located in an angular range 29 between 17° and 18°, and a diffraction peak whose maximum intensity is located in an angular range 29 between 21.3° and 22.2°, combined with a reduced Na2O content, leads to remarkable performance, despite a low CriOa content, and without the need to at least partially cover these grains with a coating containing silica.

[0028] This discovery was unexpected. The inventors were initially surprised to find that some grains exhibited significantly superior performance compared to other grains with the same chemical composition. It was only later, by comparing X-ray diffraction patterns, that they observed that the highest-performing grains all displayed the DRX1 signature according to the invention, unlike the other grains.

[0029] Furthermore, abrasive grains are advantageously usable for the abrasion of both hard steel and stainless steel.

[0030] In continuing their research, the inventors examined the potential correlation between a grain's X-ray diffraction pattern and its performance. They discovered that the DRX1 signature of the grains according to the invention could be obtained by "stabilizing" the molten material before solidification by cooling. "Stabilizing" means that the molten material must have received a minimum amount of energy before being cooled.

[0031] Tests have shown that implementing a semi-continuous melting process advantageously allows for a simple increase in the amount of energy supplied to the molten material by multiplying the number of pours.

[0032] Stabilization preferably results from the implementation of a semi-continuous melting process. This process preferably comprises a plurality of cycles, each cycle consisting of replacing a portion of the molten material bath—preferably a portion representing more than 5%, preferably more than 10%, and / or less than 35%, preferably less than 30% by mass of the bath—with a new starting charge. The mass of this new starting charge is preferably equal to the mass of the portion of the molten material bath extracted from the cell. While not bound by this theory, the inventors consider that stabilization leads to a particularly intimate mixing of the constituents, and in particular of the magnesia. According to the inventors, such an intimate mixing cannot be obtained during the usual preparation of the starting charge that forms the bath.

[0033] The minimum specific electrical energy typically depends on the composition of the melt and the furnace used. For a given composition, number of cycles, and furnace, simple tests can determine the minimum amount of energy required to stabilize the initial charges until the DRX1 signature is observed. To increase the specific electrical energy, the duration of electrical energy input, measured between feeding the furnace with a new initial charge and the partial flow of the molten material resulting from the melting of said initial charge, can also be increased.

[0034] In particular, and surprisingly, the inventors found that stabilizing the molten material bath leads to superior performance compared to Example 1 of W02004 / 094554, which was carried out without stabilization for substantially identical MgO contents, as illustrated by Examples 4 and 5 below.

[0035] A molten grain according to the first principal aspect of the invention may further exhibit one or more of the following optional characteristics:

[0036] - MgO > 0.40%, preferably MgO > 0.60%, preferably MgO > 0.70%;

[0037] - MgO < 1.60%, preferably MgO < 1.30%, preferably MgO < 0.85%;

[0038] - &2O3 < 0.15%, preferably &2O3 < 0.10%, preferably &2O3 < 0.05%;

[0039] - Na2U < 0.08%, preferably Na2U < 0.05%;

[0040] - the content of oxides other than MgO, CO₂, Na₂O and Al₂O₃ is less than 1.4%, preferably less than 1.0%, preferably less than 0.4%;

[0041] - SiU2 < 0.3%, preferably SiU2 < 0.1%, in mass percentages on the basis of oxides;

[0042] - CaO < 0.1%, preferably CaO < 0.08%, in mass percentages on the basis of oxides;

[0043] - the oxide content is greater than 96%, in mass percentages based on the mass of the melted grain;

[0044] - the RI ratio is greater than or equal to 0.05, preferably greater than or equal to 0.08;

[0045] - the RI ratio is less than or equal to 0.95;

[0046] - the molten grain is not coated, even partially, with a coating containing silica.

[0047] Beyond the 2.30% threshold, the inventors were no longer able to obtain the advantageous properties of the grains described above. However, upon further investigation, they discovered that a second DRX2 signature could be associated with advantageous properties. According to a second principal aspect of the invention, the invention thus relates to a fused grain exhibiting:

[0048] - a chemical composition such that, in mass percentages based on oxides:

[0049] 2.30% < MgO < 4.80%,

[0050] Cr2O3 < 0.2%,

[0051] Na2O < 0.1%,

[0052] Oxides other than MgO, Cr2O3, Na2O and A12O3 < 1.5%, A12O3: complement to 100%;

[0053] - a crystallographic composition characterized by an X diffraction pattern showing a "reference peak" in the angular range 29 between 18.1° and 20.1°, a first peak in the angular range 29 between 12.3° and 13.8° and, optionally, a second peak in the angular range 29 between 25.2° and 25.8°, or "corundum phase peak", each peak having a respective height, in number of shots, the ratio R2 of the height of the first peak to the height of the reference peak being greater than 0.2 and the ratio R3 of the height of the corundum phase peak to the height of the reference peak being less than 2.

[0054] Reports R2 and R3 are collectively referred to as "DRX2 signature".

[0055] This second signature can be obtained by calcining the grains at a temperature between 750°C and 1225°C.

[0056] The inventors discovered that calcination does not need to be at high temperatures, typically above 1250°C, to achieve maximum performance improvement.

[0057] Combined with a reduced Na2O content, this second signature is associated with remarkable performance, despite a low Cr2O3 content, and without the need to at least partially cover these grains with a coating containing silica.

[0058] A molten grain according to the second main aspect of the invention may also exhibit one or more of the following optional characteristics:

[0059] - MgO > 2.50%, preferably MgO > 2.70%, preferably MgO > 2.80%;

[0060] - MgO < 4.50%, preferably MgO < 4.20%, preferably MgO < 4.10%; - C CL < 0.15%, preferably C CL < 0.10%, preferably &2O3 < 0.05%;

[0061] - Na2<3 < 0.08%, preferably Na2<3 < 0.05%;

[0062] - the content of oxides other than MgO, ²⁺O₃, Na₂ <D et AI2O3 est inférieure à 1,4%, de préférence inférieure à 1,0%, de préférence inférieure à 0,4% ;

[0063] - SiC>2 < 0.3%, preferably SiCl < 0.1%, in mass percentages on the basis of oxides;

[0064] - CaO < 0.1%, preferably CaO < 0.08%, in mass percentages on the basis of oxides;

[0065] - the oxide content is greater than 96%, in mass percentages based on the mass of the melted grain;

[0066] - the R2 ratio is greater than 0.25, preferably greater than 0.3;

[0067] - the R2 ratio is less than 1.0, preferably less than 0.8;

[0068] - the R3 ratio is less than 1.8, preferably less than 0.5;

[0069] - the molten grain is not coated, even partially, with a coating containing silica.

[0070] The invention further relates to a mixture of grains comprising, by mass percentage, more than 80% of smelted grains according to the first aspect of the invention or according to the second aspect of the invention or of smelted grains selected from smelted grains according to the first main aspect of the invention and smelted grains according to the second main aspect of the invention.

[0071] The invention also relates to a method for manufacturing a mixture of molten grains according to the invention, said method comprising the following successive steps: a) mixing of raw materials so as to form a starting charge suitable for the manufacture of said mixture of grains, b) melting, preferably in a reducing medium, of said starting charge until a molten material is obtained, c) cooling of said molten material so as to solidify it entirely in less than 3 minutes, and obtain a solid mass, said solid mass from step c) being ground in a step d) so as to obtain a particulate mixture if it is not in the form of a particulate mixture.

[0072] A manufacturing process according to the invention may also have one or more of the following optional characteristics: - in step b), sufficient energy is supplied so that the solid mass has the signature DRX1;

[0073] - the process includes, subsequent to step c), in particular to obtain the DRX2 signature, a following step: f) calcination of the solid mass and / or particulate mixture, before or after an optional particle size reduction, at a temperature greater than or equal to 750°C and less than or equal to 1225°C;

[0074] - the solid mass from step c) is ground in a step d), before or after step f), so as to be in the form of a particulate mixture, the process preferably comprising a step e) of particle size selection applied to the particulate mixture;

[0075] - the process includes a said step e), step d) and step e) being preferably carried out before step f);

[0076] - in step f), the calcination temperature is greater than or equal to 800°C and less than or equal to 1200°C, preferably greater than or equal to 950°C and less than or equal to

[0077] 1150°C, and / or the calcination temperature is maintained for a period greater than or equal to 30 minutes, preferably greater than or equal to 2 hours.

[0078] Step b) is advantageously not indispensable for manufacturing grains according to the second main aspect of the invention.

[0079] Step f) is advantageously not indispensable for manufacturing grains according to the first main aspect of the invention.

[0080] In one embodiment, however, a step f) is provided in a process adapted to manufacture grains according to the first main aspect of the invention, in particular after having supplied, in step b), sufficient energy so that the solid mass exhibits the DRX1 signature.

[0081] The invention further relates to an abrasive tool comprising grains bound by a binder and agglomerated, for example in the form of a grinding wheel, or deposited on a support, for example a belt or a disc, this tool being remarkable in that at least a part, preferably more than 20%, preferably more than 30%, preferably more than 40%, preferably more than 50%, preferably more than 70%, preferably more than 80%, preferably more than 90%, by mass percentage, preferably all of said grains conform to the invention and / or are manufactured according to a process according to the invention. The abrasive tool can be a grinding wheel, and in particular a grinding wheel, a precision grinding wheel, a sharpening wheel, a cutting wheel, a solid cutting wheel, a deburring or roughing wheel, a drive wheel, a portable grinding wheel, a foundry wheel, a drill wheel, a mounted mounted wheel, a cylindrical, conical, disc or segmented grinding wheel.

[0082] The invention further relates to a method for treating a surface in hard steel, said method comprising an abrasion operation of said surface with a mixture of grains according to the invention or manufactured according to a manufacturing process according to the invention, said method being adapted to obtain the DRX1 signature and / or the DRX2 signature.

[0083] Preferably, a treatment process according to the invention comprises

[0084] - the manufacture of a grain mixture according to the invention, preferably following a manufacturing process according to the invention adapted to obtain the DRX1 signature and / or the DRX2 signature, then

[0085] - an abrasion operation of said surface with said mixture of grains, preferably after putting the mixture of grains into the form of an abrasive tool according to the invention.

[0086] The inventors also discovered that the grains according to the invention are particularly effective for the abrasion of hard steel.

[0087] This result is all the more surprising since the inventors found that the presence of MgO leads to a very clear decrease in the corundum content in the grains according to the invention, whereas as a general rule, it is considered necessary, in order to machine a hard steel surface, to use a fused grain based on corundum.

[0088] The invention also relates to the use of a mixture of grains according to the invention or manufactured according to a manufacturing process according to the invention, said process being adapted to obtain the DRX1 signature and / or the DRX2 signature, for abrading a hard steel surface.

[0089] The invention relates in particular to a kit comprising:

[0090] - a mixture of grains according to the invention or manufactured according to a manufacturing process according to the invention, said process being adapted to obtain the DRX1 signature and / or the DRX2 signature, and - a document specifying that the mixture of grains is usable for abrading a hard steel surface.

[0091] Definitions

[0092] In this description, unless otherwise stated, all compositions of a grain or mixture of grains are given as mass percentages, based on the total mass of oxides in the grain or mixture of grains.

[0093] The oxide contents of a grain according to the invention, including for "oxides other than MgO, Cr2O3, CaO and Al2O3", refer to the overall contents for each of the corresponding chemical elements, expressed in the form of the most stable oxide, according to the usual convention of the industry; therefore, sub-oxides and possibly nitrides, oxynitrides, carbides, oxycarbides, carbonitrides, or even the metallic species of the aforementioned elements are included.

[0094] By "impurities," we mean the unavoidable constituents that are necessarily introduced with the raw materials. In particular, compounds belonging to the group of oxides, nitrides, oxynitrides, carbides, oxycarbides, carbonitrides, and metallic species of silicon, sodium and other alkali metals, iron, and vanadium are impurities. Examples of sources of Al and Mg include SiCl₂, Fe₂O₃, Na₂O₃, and CaO.

[0095] By "precursor" of an oxide, we mean a constituent capable of providing said oxide during the manufacture of a grain or mixture of grains according to the invention.

[0096] A "grain" is a particle whose dimensions are all less than 20 mm.

[0097] An "alumina-based grain" is defined as a grain containing more than 85% alumina by mass, as a percentage based on oxides.

[0098] By "molten grain", or more broadly "molten product", we mean a solid grain (or product) obtained by solidification, by cooling, of a molten material.

[0099] A "molten material" is a mass rendered liquid by heating a starting charge, which may contain some solid particles, but not enough to structure the mass. To maintain its shape, a molten material must be contained within a vessel. The molten grains according to the invention are conventionally obtained by melting at temperatures exceeding 1900°C. A semi-continuous melting process is a well-known process comprising a plurality of cycles. In each cycle, a fraction of the molten material pool contained in the melting tank is extracted from the tank, and a new particulate starting charge is added to the tank, typically in such a way that the quantity of material in the tank remains substantially constant from one cycle to the next.The implementation of a semi-continuous melting process is preferred to obtain bath stabilization and thus obtain a DRX1 signature, preferably after more than 5, more than 7, more than 11 cycles.

[0100] The "median size" of a powder is defined as the size that divides the particles into first and second populations of equal mass, each consisting only of particles with a size greater than or equal to, or less than, respectively, the median size. The median size of a powder can be determined using a particle size distribution obtained with a laser particle size analyzer.

[0101] In this description, "hard steel" means steel with a Rockwell hardness of 55 HRC or higher.

[0102] Unless otherwise specified or in the event of technical incompatibility, a feature of the invention is applicable to the first and second main aspects of the invention. Detailed description

[0103] The following description is provided for illustrative purposes only and does not limit the invention.

[0104] A molten grain according to the first or second principal aspect of the invention, and preferably a mixture of such grains according to the invention, preferably exhibits one or more of the following optional and preferred characteristics:

[0105] - C CL < 0.15%, preferably CnCh < 0.10%, preferably CnCh < 0.05%, in mass percentages on the basis of oxides;

[0106] - Cr2Û3 is not voluntarily brought into the starting charge, that is to say, it is preferably an impurity;

[0107] - the Na2<3 content is less than 0.08%, preferably less than 0.05%, in mass percentages on the basis of oxides;

[0108] - the content of oxides other than MgO, CnCh, Na2 <D et AI2O3 est inférieure à 1,4%, de préférence inférieure à 1,3%, de préférence inférieure à 1,0%, de préférence inférieure à 0,9%, de préférence inférieure à 0,8%, de préférence inférieure à 0,7%, de préférence inférieure à 0,6%, de préférence inférieure à 0,5%, de préférence inférieure à 0,4%, en pourcentages massiques sur la base des oxydes ;

[0109] - oxides other than MgO and Al2O3 are preferably impurities;

[0110] - the SiCh content is less than 0.3%, preferably less than 0.2%, preferably less than 0.15%, preferably less than 0.1%, preferably less than 0.08%, preferably less than 0.05%, in mass percentages on the basis of oxides;

[0111] - the CaO content is less than 0.1%, preferably less than 0.08%, preferably less than 0.05%, in mass percentages on the basis of oxides;

[0112] - the oxide content is greater than 96%, preferably greater than 97%, or even greater than 98%, or even greater than 99%, or even greater than 99.4%, or even greater than 99.5%, or even greater than 99.6%, or even greater than 99.7%, in mass percentages based on the mass of the melted grain;

[0113] - in one embodiment, the grains are not coated, even partially, with a coating containing silica, in particular a coating containing more than 50% silica by mass;

[0114] - in one embodiment, the grains are not coated.

[0115] A molten grain according to the first principal aspect of the invention, and preferably a mixture of such grains according to the invention, preferably has one or more of the following optional and preferred characteristics:

[0116] - MgO > 0.40%, preferably MgO > 0.50%, preferably MgO > 0.60%, preferably MgO > 0.70%, preferably MgO > 0.80%, and / or preferably

[0117] MgO < 1.60%, preferably MgO < 1.40%, preferably MgO < 1.30%, preferably MgO < 1.20%, in mass percentages on the basis of oxides;

[0118] - in one embodiment, MgO < 0.87%, preferably MgO < 0.85%, or even MgO < 0.80%, in mass percentages on the basis of oxides;

[0119] - the RI ratio is greater than or equal to 0.05, preferably greater than or equal to 0.07, preferably greater than or equal to 0.08, preferably greater than or equal to 0.10;

[0120] - In one embodiment, the RI ratio is greater than or equal to 0.05, preferably greater than or equal to 0.07, preferably greater than or equal to 0.08, preferably greater than or equal to 0.10, and less than or equal to 0.95, preferably less than or equal to 0.90, preferably less than or equal to 0.85, preferably less than or equal to 0.80, preferably less than or equal to 0.70, preferably less than or equal to 0.60. A molten grain according to the second main aspect of the invention, and preferably a mixture of such grains according to the invention, preferably has one or more of the following optional and preferred characteristics:

[0121] - MgO > 2.50%, preferably MgO > 2.70%, preferably MgO > 2.80%, and / or preferably MgO < 4.50%, preferably MgO < 4.20%, preferably MgO < 4.10%, in mass percentages on the basis of oxides;

[0122] - the R2 ratio is greater than 0.25, preferably greater than 0.3, and preferably less than 1.0, preferably less than 0.9, preferably less than 0.8;

[0123] - the R3 ratio is less than 1.8, preferably less than 1.5, preferably less than 1.3, preferably less than 1.0, preferably less than 0.9, preferably less than 0.7, preferably less than 0.5, preferably less than 0.3.

[0124] Grain mixture

[0125] A mixture of grains according to the invention comprises, by mass percentages, preferably more than 85%, preferably more than 90%, preferably more than 95%, preferably more than 99%, preferably substantially 100% of melted grains

[0126] - according to the first principal aspect of the invention or

[0127] - according to the second main aspect of the invention or

[0128] - chosen from molten grains according to the first main aspect of the invention and molten grains according to the second main aspect of the invention.

[0129] Preferably, a mixture of grains according to the invention has a particle size distribution that conforms to those of the mixtures or "grits" provided by FEPA Standard 43-GB-1984, R1993 and FEPA Standard 42-GB-1984, R1993.

[0130] Preferably, a mixture of grains according to the invention has a mass retention on the 16 mm sieve, preferably on the 9.51 mm sieve, measured using a Ro-Tap® sieve-maker, of less than 1%, as a percentage by mass.

[0131] Method for manufacturing a mixture of molten grains according to the invention

[0132] Melted grains according to the invention can be manufactured by following steps a) to e), or even a) to f) mentioned above. The parameters can, for example, take the values ​​of the process used for the examples below.

[0133] In step a), raw materials are conventionally measured to obtain the desired composition and then mixed to form the starting charge. The elements Al and Mg in the starting charge are found almost entirely in the molten grains. However, the element Mg, particularly in the form of oxides, can be subject to flyaway phenomena during melting. Those skilled in the art know how to adjust the composition of the starting charge accordingly.

[0134] Choosing the raw materials for the starting charge so that the solid mass obtained at the end of step c) has a chemical analysis consistent with that of a grain according to the invention therefore poses no difficulty for a person skilled in the art.

[0135] In particular, the NaiO content must be controlled to ensure that NaiO < 0.1%.

[0136] The element Mg is preferably introduced into the starting charge in the form of MgO oxide. It can also be conventionally introduced in the form of precursors of this oxide, for example in the form of MgCCl.

[0137] The element Al is preferably introduced, at least partially, into the starting charge in the form of Al₂O₃ and / or in the form of precursors of this oxide, for example, as aluminum hydroxide and / or boehmite. Preferably, the element Al is introduced into the starting charge partly in the form of Al₂O₃ and partly in a metallic form.

[0138] In a preferred embodiment, the starting charge comprises at least one compound that creates a reducing medium during melting.

[0139] Preferably, the compound is selected from a carbon source, a metal, and mixtures thereof. Preferably, the carbon source is selected from carbon, petroleum coke, pitch, coal, and mixtures thereof, preferably petroleum coke. Preferably, the metal is aluminum.

[0140] Preferably, said at least one compound creating a reducing environment during melting and used in the starting charge is chosen from petroleum coke, aluminum and mixtures thereof.

[0141] A person skilled in the art can determine the amount of compound creating a reducing medium during melting, in the starting charge, to obtain, in step b), a melting in a reducing medium.

[0142] Preferably, the starting charge contains an amount of compound creating a reducing environment upon melting greater than 0.5%, preferably greater than 1%, preferably greater than 1.5% and, preferably less than 5%, preferably less than 4%, as a percentage by mass on the basis of the starting charge.

[0143] In step b), an electric arc furnace is preferably used, preferably of the Hérault type with graphite electrodes, but all known furnaces are conceivable, such as an induction furnace or a plasma furnace, provided that they allow the starting charge to be melted, preferably in a reducing medium.

[0144] Melting in a reducing environment is preferably obtained by the presence, in the starting charge, of compounds creating a reducing environment during melting and / or by the fact that the electrodes are immersed in the bath of molten material.

[0145] Preferably, the starting charge contains elements that create a reducing environment during melting.

[0146] Preferably, the raw materials are melted at atmospheric pressure.

[0147] Preferably, an electric arc furnace with a 70-liter chamber and a specific energy of 2 kWh or more per kg of raw materials, with a power output exceeding 220 kW, is used, or an electric arc furnace of a different capacity operated under equivalent conditions. Those skilled in the art can determine such equivalent conditions.

[0148] To "stabilize" the molten material bath, the bath can be kept molten before step c). In a semi-continuous or continuous manufacturing process, the first pours can be discarded.

[0149] In step c), cooling must be rapid, meaning that the molten material is completely solidified in less than 3 minutes. For example, this can be achieved by pouring into molds as described in US 3,993,119 or by quenching.

[0150] Preferably, the molten material is completely solidified in less than 2 minutes, preferably in less than 1 minute, preferably in less than 40 seconds, preferably in less than 30 seconds.

[0151] In one embodiment, step c) leads to a solid mass in the form of a particulate mixture.

[0152] If step c) does not directly produce a grain mixture, or if the grains do not have a particle size suitable for the intended application, the solid mass can be ground (step d)) using conventional techniques. Particle size selection (step e)), for example by sieving or cycloning, can then be carried out to obtain a grain mixture with a particle size suitable for the intended application.

[0153] Step d) and / or step e) may be carried out before or after step f) of calcination. It / they is / are preferably carried out before step f).

[0154] Preferably, the molten grains obtained at the end of step c) and / or d) and / or e) have a carbon content greater than 15 ppm and less than 1200 ppm on the basis of the mass of said grains, said content being measured using a CS744 model carbon-sulfur analyzer, marketed by the company LECO.

[0155] In step f), which is optional according to the first main aspect of the invention and obligatory according to the second main aspect of the invention, the solid mass obtained at the end of step c), preferably after reduction in the form of a particulate mixture during step d) if the process includes such a step, or preferably after step e) if the process includes such a step, is calcined.

[0156] If calcination is carried out on the solid mass directly obtained at the end of step c), a step d) and preferably a step e) is / are then carried out after step f).

[0157] The calcination is carried out at a temperature greater than or equal to 750°C, preferably greater than or equal to 800°C, preferably greater than or equal to 900°C, preferably greater than or equal to 950°C, and less than or equal to 1225°C, preferably less than or equal to 1200°C, preferably less than or equal to 1150°C, the calcination temperature being preferably maintained for a period greater than or equal to 30 minutes, preferably greater than or equal to 1 hour, preferably greater than or equal to 2 hours, preferably greater than or equal to 3 hours, preferably greater than or equal to 4 hours, and preferably less than 100 hours, preferably less than 50 hours, preferably less than 20 hours, preferably less than 15 hours, preferably less than 10 hours.

[0158] Preferably, step f) is carried out at atmospheric pressure.

[0159] Step f) can be carried out in a reducing, neutral or oxidizing atmosphere.

[0160] Preferably step f) is carried out in an oxidizing atmosphere, preferably in air.

[0161] Preferably step f) is carried out under an oxidizing atmosphere, preferably in air, preferably at atmospheric pressure, at a temperature greater than or equal to 750°C, preferably greater than or equal to 800°C, preferably greater than or equal to 900°C, preferably greater than or equal to 950°C, and less than or equal to 1225°C, preferably less than or equal to 1200°C, preferably less than or equal to 1150°C, the calcination temperature being preferably maintained for a period greater than or equal to 30 minutes, preferably greater than or equal to 1 hour, preferably greater than or equal to 2 hours, preferably greater than or equal to 3 hours, preferably greater than or equal to 4 hours, and preferably less than 100 hours, preferably less than 50 hours, preferably less than 20 hours, preferably less than 15 hours, preferably less than 10 hours.

[0162] Method for manufacturing an abrasive tool according to the invention

[0163] The manufacturing processes for abrasive tools are well known and can be used to manufacture an abrasive tool according to the invention.

[0164] Abrasive tools can in particular be formed by agglomerating grains according to the invention by means of a binder, in particular in the form of a grinding wheel, for example by pressing, or be formed by fixing grains according to the invention on a support, for example a strip or a disc, by means of a binder.

[0165] The binder can be inorganic, in particular a glass (for example, a binder made of oxides, substantially composed of silicate(s) can be used) or organic.

[0166] An organic binder is well suited. The binder can be, in particular, a thermosetting resin. It is preferably chosen from the group consisting of phenolic resins, epoxy, acrylate, polyester, polyamide, polybenzimidazole, polyurethane, phenoxy, phenol-furfural, analine-formaldehyde, urea-formaldehyde, cresol-aldehyde, resorcinol-aldehyde, urea-aldehyde, melamine-formaldehyde, and mixtures thereof.

[0167] The binder may also incorporate organic or inorganic fillers, such as hydrated (e.g., aluminum trihydrate or boehmite) or non-hydrated (e.g., molybdenum oxide) inorganic fillers, cryolite, a halogen, fluorspar, iron sulfide, zinc sulfide, magnesia, silicon carbide, silicon chloride, potassium chloride, manganese dichloride, potassium or zinc fluoroborate, potassium fluoroaluminate, calcium oxide, potassium sulfate, a vinylidene chloride-vinyl chloride copolymer, polyvinylidene chloride, polyvinyl chloride, and mixtures thereof. The binder may also contain reinforcing fibers such as glass fibers. Conventionally, a mixture of grains according to the invention is mixed with said binder optionally containing organic or inorganic fillers.The resulting mixture, in which the binder typically represents between 2% and 60%, preferably between 20% and 40% by volume, is shaped, for example, by placing it in a mold or depositing it on a support. The binder is then activated, for example by heating, to bond the grains together and / or with the optional support. After the binder has hardened and, optionally, the mixture has been demolded, an abrasive tool according to the invention is obtained.

[0168] Examples

[0169] The following non-limiting examples are given for the purpose of illustrating the invention.

[0170] Measurement protocols

[0171] The following measurement protocols were used to determine certain properties of molten grain mixtures. They allow for excellent simulation of the actual behavior of the grains when used for abrasion.

[0172] To evaluate the abrasive performance of a mixture of sample grains, a monolayer of 1.02 grams of this mixture is applied to a metal grinding wheel with a diameter of 12.7 cm, the grains being bonded together with a phenolic resin.

[0173] The surface of a 52100 hard steel plate with a Rockwell hardness between 60 and 63 HRC, measuring 20.5 cm x 7.6 cm x 6.0 cm, is then machined with the resulting grinding wheel, under water spray, using a reciprocating motion at a constant speed, maintaining a constant depth of cut of 20 µm and a grinding wheel rotation speed of 3600 rpm. The total energy developed by the grinding wheel during machining, E to t, is recorded.

[0174] After the grinding wheel has worn completely, the mass of steel machined (i.e., the mass of steel removed by the grinding operation) is measured. a and the volume of steel removed by the grinding operation "V a "

[0175] To evaluate the yield, we classically calculate the ratio S of the mass of machined steel divided by the mass of grains consumed during said machining, here equal to 1.02 grams.

[0176] To assess energy efficiency, the specific machining energy, Es, is conventionally calculated, equal to the energy required to remove a unit volume of steel (Es = E to TV a ). To determine the composition of the molten grains, a bead of a mixture of these grains is made by melting the mixture, then chemical analysis is carried out by X-ray fluorescence, except for the measurement of carbon content.

[0177] The carbon content of the molten grains in the examples is measured using a CS744 model carbon-sulfur analyzer, marketed by the company LECO.

[0178] The median size of a powder is classically measured using a LA950V2 laser particle size analyzer marketed by the company Horiba.

[0179] The X-ray diffraction pattern of the molten grains in the examples is produced on a powder of particles that have passed through a sieve with an opening of 40 pm, obtained by grinding said grains.

[0180] X-ray diffraction pattern acquisition is performed using a Bruker D8 Endeavor instrument equipped with a copper anode, over an angular range of 5° to 100°, with a step size of 0.01° and a counting time of 0.34 s / step. The front optics consist of a 0.3° primary slit and a 2.5° Soller slit. The sample is rotated at a speed of 5 rpm using the automatic cutter. The rear optics consist of a 2.5° Soller slit, a 0.0125 mm nickel filter, and a 1D detector with a 4° aperture.

[0181] The diffraction patterns are then qualitatively analyzed using the DIEERAC.EVA software marketed by Bruker and the PDE5 + 2024 database.

[0182] For the fused grains according to the first principal aspect of the invention, each surface required for the calculation of the RI ratio is selected in the X diffraction diagram using the "select area" function, and the value of the surface is the "net area" value given by the DILLRAC.EVA software.

[0183] Manufacturing protocol

[0184] The mixtures in the examples were prepared using the following raw materials:

[0185] - an alumina powder of purity greater than 99.8% by mass, containing the impurities NaiO, CaO, EeiCh, MgO, TiCl, SiCl, and having a median size equal to 80 pm;

[0186] - a magnesia powder, of purity greater than 99% by mass, of which more than 85% of the grains, by mass, pass through the mesh of a 45 µm sieve. Reference example 1 (“Ref”), outside the scope of this invention, is a mixture of molten grains marketed by Saint-Gobain Speciality Grains and Powders under the name MA88K.

[0187] The grain mixtures of Examples 2 to 9 were prepared according to the following manufacturing process: a) mixing of the raw materials to form a starting charge suitable for the manufacture of the grain mixture to be obtained, said starting charge comprising 2% metallic aluminum chips and 0.5% petroleum coke, in mass percentages based on said starting charge, the mass of a starting charge being equal to 25 kg, b) discharge of said starting charge into a single-phase electric arc furnace of the Hérault type with graphite electrodes, with a furnace pit of 0.8 m diameter, and melting of said starting charge in a reducing medium with a voltage of 150 V, a current between 1700 A and 1800 A and a specific electrical energy as described in Table 1, c) partial emptying of the melting furnace of a quantity of molten material substantially equal to the mass of the starting charge,pouring and rapid cooling of the molten material, so that said molten material is completely solidified in less than 3 minutes, by means of a casting device between thin metal plates such as that shown in US patent 3, 993,119, so as to obtain a completely solid mass in the form of a plate, the details of the melting cycles carried out being shown in Table 1 below, d) for each melting cycle (a)+b)+c)), grinding of said solid mass cooled in step c) so as to reduce it to the form of a mixture of grains, e) for each melting cycle, selection by sieving using a Ro-Tap® sieve-maker of grains having a size between 500 and 600 µm.,

[0188] Before the first melting cycle, the furnace tank contained 180 kg of a partially liquid raw material mixture, suitable for manufacturing a grain mixture according to the invention, the furnace being in operation and having received, before the introduction of the first starting charge, a specific electrical energy equal to 2.7 kWh / kg of said raw material mixture.

[0189] [Table 1]

[0190] (1): Number of successive melting cycles to obtain the grain mixture of the example; a cycle consists of loading a starting charge into the furnace, melting it, pouring by partially emptying the furnace, the molten material extracted from the vessel being cooled;

[0191] (2) Average specific electrical energy supplied during each melting cycle (kWh / kg loaded during the cycle);

[0192] (3) Time during which said specific energy is applied during each fusion cycle (h);

[0193] (4) Total amount of electrical energy supplied to all starting loads to obtain the grain mixture in the example (kWh).

[0194] For example, the grain mixture in Example 7 was obtained at the end of the twelfth melting cycle, the total quantity of starting feed introduced into the melting furnace being equal to 12x25=300 kg, the total quantity of electrical energy supplied being equal to 2.5x300=750 kWh, and the total time during which said specific electrical energy was applied being equal to 12x0.28=3.36 h.

[0195] When calcination was carried out, it was performed under air, at atmospheric pressure, at temperature T, temperature T being maintained for a time t, the rate of rise to temperature T being equal to 300°C / h.

[0196] Table 2 below provides the chemical composition of these grain mixtures, their crystallographic characteristics, the temperature T, the time t, and the results obtained with said mixtures. The percentage improvement in the S ratio is calculated using the following formula: 100 x (S ratio of the mixture in the example under consideration - S ratio of the mixture in reference example 1) / S ratio of the mixture in reference example 1.

[0197] A high, positive percentage improvement in the S ratio is desired, without a significant increase in specific energy, and preferably with a decrease in specific energy (the positive percentage reduction in specific energy Es described below). The inventors consider a percentage improvement in the S ratio greater than 5% to be significant.

[0198] Preferably, the S ratio is improved by more than 10%, preferably by more than 20%, preferably by more than 30%, preferably by more than 40%, preferably by more than 50%, preferably by more than 60%, preferably by more than 70%, preferably by more than 80%.

[0199] The percentage reduction of specific energy, Es, is calculated by the following formula: 100x(Es with the mixture of reference example 1 - Es with the mixture of the example under consideration) / Es of the mixture of reference example 1.

[0200] The inventors consider a reduction of more than 5% in specific energy Es to be significant. Preferably, the specific energy is reduced by more than 10%, preferably by more than 15% (compared to the reference).

[0201] [Table 2]

[0202] (*) : outside invention nd: not determined A comparison of examples 1 (reference example), 2 according to the invention and 3 according to the invention shows the positive impact of the presence of 0.51% MgO and the DRX1 signature: the grain mixture of example 2, not calcined, shows a percentage improvement in the positive S ratio of 84% and a reduced specific energy of 4%, and the grain mixture of example 3, calcined at a temperature T equal to 1000°C, for a time t equal to 4 hours, shows a percentage improvement in the positive S ratio of 122%, without degradation of the specific energy.

[0203] A comparison of examples 1 (reference example), 4 (outside the invention), and 5, 6, and 7 (according to the invention) shows the impact of stabilization during melting and the DRX1 signature for MgO contents of 0.91%, 0.94%, 0.94%, and 0.92%, respectively: the grain mixture of example 4, uncalcined and lacking the DRX1 signature, exhibits a 15% improvement in the positive S ratio and a 35% increase in specific energy, in other words, a 35% degradation in specific energy. In contrast, the grain mixtures of examples 5, 6, and 7, uncalcined, calcined at a temperature T of 1000°C for a time t of 6 hours, and calcined at a temperature T of 1000°C for a time t of 4 hours, respectively, exhibiting the DRX1 signature, show a positive S ratio improvement. of 64%, 88%, and 145%, respectively, without degradation of specific energy for the grain mixture of examples 5 and 6,and a reduced specific energy of 21% for the mixture in example 7.

[0204] A comparison of example 2 with example 3, or of example 5 with examples 6 and 7, shows the positive impact of calcination.

[0205] A comparison of examples 1 (reference example) and 8 according to the invention shows the positive impact of the presence of 2.21% MgO and the DRX1 signature: the grain mixture of example 8, calcined at a temperature T equal to 1000°C for a time t equal to 6 hours, shows a percentage improvement in the positive S ratio of 81% and a reduced specific energy of 6%.

[0206] A comparison of examples 1 (reference example) and 9 (outside the invention) shows that the grain mixture of example 9, uncalcined, having a MgO content of 2.56% and exhibiting the DRX1 signature, shows a percentage improvement in the positive S ratio of 45%, and a specific energy increased by 13%.

[0207] As is now clear, the invention provides a mixture of alumina-based molten grains exhibiting better performance and energy efficiency than the known alumina-based molten grains of the reference example for abrading a hardened steel surface, even in the absence of calcination. They are also effective for abrading a stainless steel surface.

[0208] Of course, the present invention is not limited to the embodiments described, which are provided by way of illustrative and non-limiting examples.

[0209] In particular, the molten grains according to the invention are not limited to specific shapes or dimensions. They could be used in applications other than the abrasion of a hardened steel surface.

[0210] As is now clear, the invention provides a mixture of alumina-based fused grains exhibiting better performance and energy efficiency than the known alumina-based fused grains of the reference example for the abrasion of a hardened steel surface. Of course, the present invention is not limited to the described embodiments provided by way of illustrative and non-limiting examples.

[0211] In particular, the molten grains according to the invention are not limited to specific shapes or dimensions. They could be used in applications other than the abrasion of a hardened steel surface.

Claims

DEMANDS 1. Melted grain exhibiting - a chemical composition such that, in mass percentages based on oxides: 0.30% < MgO < 2.30%, Cr2O3 < 0.2%, Na2O < 0.1%, Oxides other than MgO, Cr2O3, Na2O and A12O3 < 1.5%, A12O3: complement to 100%; - a crystallographic composition characterized by a diffraction pattern X exhibiting a first peak in the angular range 29 between 17° and 18° and, optionally, a second peak in the angular range 29 between 43° and 44°, or "corundum phase peak", each peak having a respective area, in strokes x degrees, the ratio RI of the area of ​​the first peak to the sum of the area of ​​the first peak and the area of ​​the corundum phase peak being greater than 0.

02.

2. Melted grain according to the preceding claim, in which - MgO > 0.40%, and / or - MgO < 1.60%, and / or - Cr2O3 < 0.15%, and / or - Na2O < 0.08%, and / or - the content of oxides other than MgO, Cr2O3, Na2O and Al2O3 is less than 1.4%.

3. Melted grain according to the immediately preceding claim, in which - MgO > 0.60%, and / or - MgO < 1.30%, and / or - Cr2O3 < 0.10%, and / or - Na2O < 0.05%, and / or - the content of oxides other than MgO, Cr2O3, Na2O and Al2O3 is less than 1.0%.

4. Melted grain according to the immediately preceding claim, in which - MgO > 0.70%, and / or - MgO < 0.85%, and / or - Cr2Û3 < 0.05%, and / or - the content of oxides other than MgO, β2O3, Na2U and Al2O3 is less than 0.4%.

5. Melted grain according to any one of the preceding claims, in which - SiU2 < 0.3%, in mass percentages based on oxides, and / or - CaO < 0.1%, in mass percentages on the basis of oxides.

6. Melted grain according to the immediately preceding claim, in which - SiU2 < 0.1%, in mass percentages based on oxides, and / or - CaO < 0.08%, in mass percentages on the basis of oxides.

7. Melted grain according to any one of the preceding claims, wherein the oxide content is greater than 96%, in mass percentages on the basis of the mass of the melted grain.

8. Melted grain according to any one of the preceding claims, wherein the RI ratio is greater than or equal to 0.

05.

9. Melted grain according to the immediately preceding claim, wherein the RI ratio is greater than or equal to 0.

08.

10. Melted grain according to any one of the preceding claims, wherein the RI ratio is less than or equal to 0.

95.

11. Melted grain according to any one of the preceding claims, said grain not being coated, even partially, with a coating comprising silica.

12. Grain mixture consisting of smelted grains according to any one of the preceding claims.

13. A method for manufacturing a molten grain mixture according to the immediately preceding claim, said method comprising the following successive steps: a) mixing raw materials to form a starting feed suitable for manufacturing said grain mixture, b) melting said starting feed, preferably in a reducing medium, until a molten material is obtained, c) cooling said molten material to solidify it entirely in less than 3 minutes, and obtain a solid mass, said solid mass from step c) being ground in a step d) so as to obtain a particulate mixture if it is not in the form of a particulate mixture, step b) being adapted so that said solid mass has an RI ratio greater than 0.

02.

14. A method according to the immediately preceding claim, wherein step b) implements a semi-continuous method.

15. A process according to the immediately preceding claim, wherein the semi-continuous process comprises more than five cycles.

16. A method according to any one of the three immediately preceding claims comprising a step e) of particle size selection applied to the particulate mixture.

17. A process according to any one of the four immediately preceding claims, comprising a step f) of calcining the solid mass and / or the particulate mixture at a temperature greater than or equal to 750°C and less than or equal to 1225°C.

18. A method according to any one of the two immediately preceding claims, wherein steps d) and e) are carried out before step f).

19. Abrasive tool containing grains - bound together by a binder and agglomerated or - deposited on a support, at least a part of said grains conforming to any one of claims 1 to 11.

20. Abrasive tool according to the immediately preceding claim, comprising more than 30%, by mass percentage, of grains according to any one of claims 1 to 11.

21. Abrasive tool according to any one of the two immediately preceding claims, in the form of a grinding wheel, a belt or a disc.

22. A method for treating a surface in hard steel, said method comprising an operation of abrasion of said surface with a mixture of grains according to claim 12 or manufactured according to a method according to any one of claims 13 to 18.

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